Of the known methods for synthesis of sodium aluminum hydride (NaAlH.sub.4), the so-called "direct synthesis" process appears to offer the most promise. It involves reacting Na (or NaH), Al and H.sub.2 under suitably elevated temperatures and pressures in an appropriate liquid reaction medium, such as an ether, amine or hydrocarbon. See in this connection:
E. C. Ashby, Chem. Ind. (London), 208 (1962); PA1 E. C. Ashby et al., Inorg. Chem. 499 (1963); PA1 E. C. Ashby, French Pat. 1,235,680 (May, 1960), and U.S. Pat. Nos. 4,045,545 and 4,081,524; PA1 J. Murib, U.S. Pat. No. 3,556,740; PA1 H. Clasen, Angew. Chem. 73, 322 (1961).
Among the drawbacks of the direct synthesis process are the relatively long induction periods that are usually encountered even when using activated aluminum. And, because of the sensitivity of Na (and NaH) to water and alcohols, it has been standard practice to carefully pre-dry or otherwise purify the liquid reaction medium before use. For example, it has been customary to dry the hydrocarbon media (e.g., toluene, hexane, trimethylhexane) over Na and to distill the ether media (e.g., diglyme, tetrahydrofuran, diethyl ether) over NaAlH.sub.4 just prior to use.
Snyder, U.S. Pat. No. 3,387,949 discloses a process for making alkali metal hydrides by reaction of the metals and hydrogen. The process is carried out in the presence of a catalyst and the reactants and products are comminuted during the course of the reaction. The reference discloses additional comminution conducted before the reaction mixture is pressured with hydrogen.